*2.4. Microhardness, Modulus of Elasticity, and Adhesive Bond Strength of DLCs*

We used the method of nanoindentation with a Berkovich diamond indenter on CSEM Nano hardness tester (CSM Instruments, Needham, MA, USA) equipped with specialized software (version 4.0) based on the algorithm of Oliver and Pharr [26,56]. This method, in contrast to the Vickers pyramid microhardness estimation scheme, is more reliable, since, if necessary, it allows excluding the influence of the hardness of the underlying layers on the results of measurements of the characteristics of the outer DLC layer. Before starting the measurements, the equipment was calibrated on reference samples with the known modulus of elasticity and hardness.

In experiments, the value of the applied load and the corresponding depth of indenter insertion were selected based on the condition of not more than 15% of the thickness of the DLC. The measurements were performed at loads of 1.0 and 4.0 mN, and the corresponding maximum indenter insertion depths were 0.09 and 0.38 μm, respectively. The tests were carried out as follows: after the load on the coated sample reached the maximum value, unloading began, and the load acting on the indenter gradually decreased to zero. The duration of the load-unload cycle was 50 s. According to the measurement results, typical experimental curves of continuous indentation were obtained—"the dependence of the load on the depth of indentation". The first curve corresponds to loading and reflects the resistance of the material to the penetration of the indenter. The second describes the return of deformation after

the removal of the external load and characterizes the elastic properties of the material. In this case, the deformation response (indentation depth) is simultaneously recorded in nanometer resolution.

The obtained experimental data allowed us to not only judge the nano hardness (*H*) but also the modulus of elasticity (*E*) of the DLC. If we obtain a quantitative value of the *H*/*E* ratio (index of plasticity) [57–59], we can approximate the viscosity of the coating and its ability to resist possible deformation and destruction during cutting. For reducing possible measurement errors, each sample was processed five times, resulting in an average hardness value.

To quantify the strength of the adhesive bond of DLCs to carbide substrates, we used the CSEM micro scratch tester system. It implements the method of scratching with a diamond cone indenter (apex angle of 120◦ and apex radius of 100 μm) with a variable load from 1 to 30 N. The spectra of the acoustic emission signal were constantly recorded. Their bursts make it possible to reliably judge the initial stage of cracking and subsequent peeling of the coatings. Hard-alloy samples with coatings moved at a constant speed, and the recording and processing of acoustic emission spectra were carried out using specialized software (version 4.0). Thus, when conducting scratch tests, is solved the main problem of determining the critical load, at which there is an abnormal change in the indentation depth of the indenter and the separation of the coating.

In addition to the method described above, a qualitative assessment of the adhesive bond strength of the coating to the substrate was performed by pressing the Rockwell indenter at a load of 15 N/mm<sup>2</sup> on a Wilson Hardness R574T device by INSTRON (Norwood, MA, USA). In this case, a comparative assessment of the adhesion characteristics of two types of coatings was made by comparing the prints obtained after the intrusion of a cone indenter.

### *2.5. Friction Coe*ffi*cient and Abrasion Resistance of DLC Coatings*

The friction coefficient was evaluated in work on a Tetra Basalt N2 testing machine from TETRA GmbH to assess the DLC's effect on the transformation of frictional properties. During the tests, the coefficient of friction-sliding of rubbing pairs "hard alloy with DLC-coating—a counter body made of AlCuMg2 alloy" and "hard alloy with DLC-coating—a counter body made of 41Cr4 steel" were determined. The tests of all samples were carried out under conditions of dry friction at identical normal loads on the counter body (1 N), with the speed of relative displacement being 2 mm·s−<sup>1</sup> and the friction path being 90 mm. We evaluated the coefficient of friction for samples with single-layer DLCs.

The abrasion resistance of the surface layer of DLC-coated carbide samples was carried out by testing on a Calotest unit by CSM Instruments. The test principle was that a rotating sphere was placed on a test sample and operated with a preselected load of 20 N, which was controlled by a particular sensor. The rotation of the sphere during the tests was carried out by the driveshaft, and the position of the sphere to the test samples and the load in the contact area was constant. A water-based abrasive suspension was fed to the test area, and its solid particles in the contact area of the sphere with the coated sample led to abrasion of the surface area and the formation of a wear hole (spherical recess). The suspension and sphere wear the coating and substrate in a controlled manner, which guarantees reproducible results. Optical analysis of the geometric dimensions of the worn hole makes it possible to judge the ability of hard-alloy samples with DLCs to resist abrasion [60,61].
